The present invention relates to an improved kaolin clay product useful in the paper industry and, more particularly, to a blended kaolin clay slurry shippable at a high solids content and having a better high shear viscosity than a neat slurry at similar solids content of either of the individual kaolin clays making up the blend.
Large amounts of refined kaolin clay are used in the paper industry, at times in combination with calcium carbonate as a coating for paper to produce a high quality surface for printing. Although the qualities of brightness, gloss and opacity are of prime importance in clay slurries produced for use as paper coatings, the clay slurries must also have acceptable rheological properties so that they may be readily blended with calcium carbonate and other materials, such as starch and/or latex binders, to form the ultimate paper coating. Heretofore, it has been customary to ship kaolin clay slurries for use in paper coating at solids levels in the range of about 69.5% to 70.5%.
In conventional practice within the paper industry, slurried calcium carbonate is blended with the 69.5% to 70.5% solids slurried kaolin together with starch and/or latex binders to form a paper coating which is spread over the paper to improve the print gloss of the paper and provide a high quality surface for printing. In the process of blending these materials, the solids content of the resultant coating is typically reduced to a level in the range of 55% to 65% solids, which is lower than the desired level of solids that some segments of the paper industry would prefer to see in paper coating of 65% to 72% by weight.
In order to raise the solids level in the resultant paper coating to the desired level within the industry, dry kaolin clay must be added to the paper coating mixture. Typically, the dry kaolin is either added to the mixture from bags through a very labor intensive hand operation or through elaborate and capital intensive bulk solids handling systems. Of course, the amount of dry kaolin particulate solids which can be added to the paper coating may be limited in any given instance by resulting increases in the viscosity of the paper coating above acceptable limits and the high energy costs associated with the high shear mixing required to thoroughly distribute the dry solids throughout the paper coating.
Accordingly, it would be desirable to provide to the papermaking industry a slurried kaolin product having a solids preferred level in the range of 72% to 76%, i.e. a solids level approximately equivalent to that of the slurried calcium carbonate typically mixed with the slurried kaolin to form the paper coating, so that upon addition of the low solids latex. and starch binders, the resultant paper coating will have a solids content in the preferred range of 65% to 72% solids without the labor, energy and capital investment cost associated with adding dry kaolin to the paper coating blend. However, prior attempts to produce a slurried kaolin product at a solids content on the order of 75% have been limited as the viscosity of kaolin slurries resulting from these prior art attempts have resulted in unacceptably poor low and high shear viscosities at solids levels above 70%.
Accordingly, it is an object of the present invention to provide a slurried kaolin clay product having a solids content of at least 72% by weight while having rheological properties acceptable for use of the product as a paper coating.